Field
The presently disclosed subject matter relates to an optical deflector used in an optical scanner for a projector, a laser headlamp, a bar code reader, a laser printer, a laser head amplifier, a head-up display unit and the like.
Description of the Related Art
Recently, optical deflectors used in optical scanners have been micro electro mechanical system (MEMS) devices manufactured by semiconductor manufacturing technology and micro machine technology.
A first prior art optical deflector is constructed by a mirror supported by torsion bars to a support frame. Also, provided between the support frame and the torsion bars are actuators serving as cantilevers. Thus, the mirror can be rocked around an axis by the actuators.
In the above-described first prior art optical deflector, since the thickness of the mirror is the same as that of the torsion bars, the mirror is very thin. Therefore, the moment of inertia of the mirror is so small that the resonant frequency of the mirror is very large. As a result, the mirror can be driven at a higher speed than a required speed.
In the first prior art optical deflector, however, since the mirror is very thin, the rigidity of the mirror is very small. Therefore, when the rocking angle of the mirror is large, a relatively large stress as a repulsive force would be spread isotropically and broadly into the mirror from the torsion bars (see: FIG. 5A). As a result, the entire mirror would be greatly deformed in a bowl shape, so that the dynamic face-deflection peak-to-valley amount would be very large. Thus, the optical scanning characteristics of reflected light of the mirror would not satisfy the required optical scanning characteristics in optical scanners for high definition projectors. At worst, the mirror entirely would break down. Note that the required dynamic face-deflection peak-to-valley amount is defined by one-tenth of a wavelength (λ=450 nm) of a laser beam irradiated onto the mirror.
A second prior art optical deflector is further constructed by a ring-shaped reinforcement rib provided on a rear surface of the mirror of the first prior art optical deflector. In this case, the size of the ring-shaped reinforcement rib is smaller than that of the mirror. Therefore, the substantial thickness of the mirror is larger than that of the torsion bars (see: FIG. 11 of US2014/0071512A1).
In the above-described second prior art optical deflector, due to the presence of the ring-shaped reinforcement rib, the rigidity of the mirror is larger than that of the mirror of the first prior art optical deflector. Therefore, when the rocking angle of the mirror is large, a relatively large stress as a repulsive force spread from the torsion bars into the mirror would be interrupted by the ring-shaped reinforcement rib (see: FIG. 5B). In other words, no substantial stress occurs in a central portion of the mirror within the ring-shaped reinforcement rib. As a result, the dynamic face-deflection peak-to-valley amount of the mirror is smaller than that of the first prior art optical deflector.
In the above-described second prior art optical deflector, however, the above-mentioned relatively large stress would still broadly spread into portions of the mirror between the torsion bars and the ring-shaped reinforcement rib. Therefore, the portions of the mirror between the torsion bars and the ring-shaped reinforcement rib would be distorted, so that the optical scanning characteristics of reflected light from the mirror would still deteriorate. Additionally, the ring-shaped reinforcement rib would be peeled off.
In a third prior art optical deflector, protruded portions are provided at the mirror of the second prior art optical deflector along a rocking direction in the vicinity of a coupling portion between the mirror and the torsion bars, and extension portions of the reinforcement rib are coupled to the protruded portions of the mirror (see: WO2014/122781A1).
In the above-described third prior art optical deflector, due to the presence of the extension portions of the ring-shaped reinforcement rib, the rigidity of the mirror is larger than that of the mirror of the second prior art optical deflector. Therefore, when the rocking angle of the mirror is large, a relatively large stress as a repulsive force spread from the torsion bars into the mirror would also be interrupted by the extension portions of the ring-shaped reinforcement rib (see: FIG. 5C). In other words, no substantial stress occurs in portions of the mirror beyond the extended ring-shaped reinforcement rib in addition to the central portion of the mirror. As a result, the dynamic face-deflection peak-to-valley amount of the mirror is smaller than that of the second prior art optical deflector.
In the above-described third prior art optical deflector, however, the above-mentioned relatively large stress would still spread into portions of the mirror surrounded by the extension portions of the ring-shaped reinforcement rib. Therefore, the portions of the mirror surrounded by the extension portions of the ring-shaped reinforcement rib would be distorted, so that the optical scanning characteristics of reflected light from the mirror would still deteriorate. Additionally, the ring-shaped reinforcement rib would be peeled off.